1. Field of the Invention
The present invention relates to a zoom lens that is suitable for use in an image pickup apparatus such as a digital still camera, video camera, and TV camera.
2. Description of the Related Art
As taking optical systems for use in image pickup apparatuses, small zoom lenses having a high zoom ratio with a zoom range covering wide angles, high optical performance and short overall length are demanded. When used in a camera using a solid state image pickup element, a zoom lens is required to have a relatively long back focus, because various optical components such as a low pass filter and a color correction filter are provided between the rear end of the lens and the image pickup element in such a camera. Moreover, when used in an image pickup apparatus using an image pickup element for color images, a zoom lens is desired to have good telecentricity on the image side in order to prevent color shading.
Negative-lead zoom lenses in which a lens unit having a negative refractive power is located closest to the object side are known as zoom lenses that are small in overall size, have a long back focus and good telecentricity on the image side.
There is a known three-unit, negative-lead zoom lens including, in order from the object side to the image side, a first lens unit having a negative refractive power, a second lens unit having a positive refractive power, and a third lens unit having a positive refractive power. There is a known three-unit zoom lens of this type that is telecentric on the image side and has a long back focus and a wide field angle (as disclosed in, for example, U.S. Pat. No. 7,920,334 and United State Patent Application No. 2010/0091381).
In the three-unit zoom lens, in order to achieve high optical performance throughout the entire zoom range while making the overall size of the system small, it is important to arrange the refractive powers and lens configurations of the lens units appropriately. In particular, in order to achieve excellent optical performance with reduced curvature of field throughout the entire zoom range, it is important to appropriately select the refractive power of the first lens unit, the refractive powers and materials of the positive lenses and negative lenses used in the zoom lens system. For example, in order to achieve a flat image field throughout the entire zoom range, it is important to make the Petzval sum small. To this end, it is effective that a plurality of positive lenses be arranged to contribute to the positive refractive power of the entire system and that the positive lenses each made of a material having a high refractive index be used.
In the embodiments disclosed in U.S. Pat. No. 7,920,334, the average of the refractive indices of the materials of the positive lenses in the entire system is in a range of 1.65 to 1.71. In the case of United State Patent Application No. 2010/0091381, the average of the refractive indices of the materials of the positive lenses is in a range of 1.70 to 1.77 or less. If the refractive power is increased without increasing the number of positive lenses in the lens systems disclosed in these documents with a view to reduce the overall size, an increase in the Petzval sum will result, thus leading to strong curvature of field. In particular, if high zoom ratio and small overall size are to be achieved, it will be very difficult to achieve a flat image field throughout the entire zoom range.
An object of the present invention is to provide a zoom lens that is small in overall size and has high optical performance throughout the entire zoom range and to provide an image pickup apparatus equipped with such a zoom lens.